One major problem with vehicles that burn fossil fuels, such as gasoline and diesel, is air pollution. To solve such a problem, drawing attention is a technology in which a rechargeable secondary battery is utilized as a power source of vehicles. Accordingly, electric vehicles (EV) that are operable merely by a secondary battery, and hybrid-electric vehicles (HEV) that use both a secondary battery and an internal combustion engine, have been developed, and some of them are commercialized. A representative example of the secondary battery, which is usable as a power source of EVs and HEVs, is a nickel-metal hybrid (Ni-MH) battery, but the use of lithium-ion batteries has also been tried recently.
To use the secondary battery as a power source of EVs and HEVs, the secondary battery must have a high output and capacity. Considering a conventional configuration to satisfy this requirement, a plurality of small secondary batteries (battery cells) is interconnected in series or in parallel to constitute a battery module, and in turn, a plurality of battery modules is interconnected in series or in parallel to constitute a single battery pack.
However, the high output and capacity secondary battery has a problem in that it generates a large amount of heat in a charge/discharge mode thereof. If the heat of battery cells generated in a charge/discharge mode is not effectively removed, the heat is accumulated, resulting in a degradation of the battery cells. For this reason, it is necessary to provide such a high output and capacity battery back with a cooling system.
As examples of a system for cooling a battery pack according to the prior art, U.S. Pat. No. 5,589,290 and Korean Patent Laid-Open Publication No. 2004-45937 can be referred. Now, the principle of a conventional system for cooling a battery pack will be explained with reference to FIG. 1.
As shown in FIG. 1, the conventional battery pack cooling system, designated as reference numeral 10, includes a battery pack 20 having a plurality of batteries, a refrigerant introduction section 30 arranged at a lower end surface of the battery pack 20, and a refrigerant discharge section 40 arranged at an upper end surface of the battery pack 20. The battery pack 20 consists of a plurality of battery modules 50 electrically connected to each other. Also, each of the battery modules 50 consists of a plurality of battery cells 60 electrically connected to each other. The battery cells 60 of each battery module 50 define slight gaps in contact regions of the neighboring battery cells 60, so that a refrigerant, introduced via the introduction section 30, is used to dissipate heat generated from the battery cells 60 while moving through the gaps. The used refrigerant, after that, is discharged via the discharge section 40 provided at the top of the battery pack 20.
However, the battery pack cooling system 10 having the above-described configuration as shown in FIG. 1 has several problems as follows.
Firstly, in the case of the conventional battery pack cooling system 10, it is difficult to uniformly distribute the refrigerant, having passed through the introduction section 30, into the respective battery modules 50. This inevitably results in a wide temperature difference between the battery cells 60. Recent researches confirm that such a wide temperature difference between the battery cells 60 is a main factor of degrading the overall performance of the battery pack 20.
Secondly, since the refrigerant introduction section 30 and the refrigerant discharge section 40 are independently arranged at the upper and lower sides of the battery pack 20, it is necessary to arrange two refrigerant guide members at both the upper and lower sides of the battery pack 20, respectively. This doubles a space for the installation of the refrigerant guide members, and thus, undesirably increases the size of an overall battery system.
For these reasons, most currently commercialized battery pack cooling systems are unable to provide optimal temperature control of battery cells, and suffer from an increase in the size of an overall battery system.